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Energy:
Concepts and Examples
SHORT VERSION – may look weird
Steve Colt
Envi 201
University of Alaska Anchorage
[email protected]
Version 1: 16 Feb 2004
Energy: What is It?
• Ability to do Work
– (formal physics definition)
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Food  We can move, think, live
Warmth or Heat, Coolness
Light
Sound (speaker)
Motion (transportation)
Mechanical Power (skilsaw)
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What Forms does it Take?
• Chemical
– Bagel, gasoline
• Heat
– Warm room, hot water
• Mass (E=Mc2)
– Sun, nuclear bomb, nuclear reactor
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Forms of Energy
• Kinetic – energy of motion
– Speeding bicycle
• Potential – energy of position
– Bicycle at top of hill
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Forms of Energy
• Electric Energy (Charge)
– Charged Battery
• Electromagnetic Radiation
– Visible Light, microwaves
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Measuring Energy – Common Units
• Joule – metric unit (not very useful)
– 1 bagel has 1.2 million joules of food
energy
• Btu – British thermal unit
– 1 lb water up by 1 degree f
• kWh – kilowatt hour
– Used for electricity – 1 kWh costs 10
cents in Anchorage
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Power: the Rate of Energy Use
– Just as speed = distance per time,
• Power = Energy conversion per unit of
time
• Measured as Watts (W)
– 60-watt light bulb converts electricity
to light (and heat!)…
– While it is on!
– Technically, 1W = one joule per second
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Power: Examples
• Human (at rest)
– About 100 W (we are always on!)
• Small candle
– About 1 W (when it is lit!)
• Hair dryer
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1,000 W = 1 kilowatt = 1 kW
If it is on for one hour it uses:
1 kW x 1 hour = 1 kilowatt-hour (1 kWh)
(costs 10 cents)
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Power: Horse vs. Hairdryer
• 1 Horsepower = .67 kW = 670 watts
– Invented by James Watt so he could
charge for steam power in a way people
could understand
– Few horses can actually produce 1 hp
– Hairdryer, using 1 kilowatt, = 1.3
horsepower
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Conversion Example: Jet Engine
Forms:
Picture of jet engine
Chemical -> Heat ->
Motion
Power output:
15 Megawatts
during take-off,
Much less during
cruise
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Energy Flow in Natural Systems
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Solar Radiation Received at Earth’s Surface:
average is about 150 watts per square meter
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Conversion of Solar to Plant
Biomass
Only about 1-5% of
solar radiation is
converted to
chemical energy of
plant matter
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Conversion of Plants to Animals
• Trophic Level = feeding level
• Plants (autotrophs = self-feeders)
• Herbivores
– Convert 1-10% of plant energy to
biomass
• Carnivores
– Convert 1-10% of herbivore energy to
their own biomass
• 16 lbs of corn = 1 pound of beef
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Two Laws of Thermodynamics
• First Law:
– Energy is converted from one form to
another
• Second Law:
– Energy is degraded with every
conversion, and ends up as low-grade
heat
• Example: Your Toaster
– …Electricity->Heat->(toast + warmth)
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Implication:
• Energy conversions are never 100%
efficient
• There is waste heat lost at every
step
• Efficiency Varies Dramatically…
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Efficiency of Energy Conversion
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Pre-Industrial Energy Sources
Source: Smil 1994
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Energy Use per Person
Source: Smil 1994
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Energy Use per Person
• The U.S. converts energy at the
constant rate of about 10 kW per
person
• This is like having 100 “energy slaves”
working full-time for every american
child and adult person
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Energy Use vs. Human Development
The Human
Development Index
combines life
expectancy,
education, and
income into an index
that ranges from zero
to 1
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Impacts: Sulfur and Nitrogen
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Impacts: Land Use from Production
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Changing Energy Efficiency
Light Bulbs
Jet Engines
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Overall, we seem to be getting
more efficient…..
This is the “T” factor
– how much energy
per dollar of
economic output
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But World energy use still grows
by 1-2%/yr
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References
• Smil, Vaclav. 1994 Energy in World History. Westview.
• Smil, Vaclav. 2003. Energy at the Crossroads. MIT Press.
• Scott Brennan and Jay Withgott. 2003. Environment: The
Science Behind the Stories. Pearson/Cummings.
• BP Statistical Review of World Energy. 2003.
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